Trim and clean the mushrooms. Shitakes need stems removed. Portobellos need stems removed, and the black gills scraped off with a spoon so they won't blacken the entire mixture. Lobster mushrooms need a good washing in water. The small mushrooms are good left whole. Bigger ones are best torn, or cut into slices or chunks. Keep it rustic.
While doing that, melt the butter in the oil in a large skillet on medium-high heat. When butter is sizzling, add the mushrooms into pan. Toss, and then let sit and cook, tossing every couple of minutes for about 6 to 8 minutes. Add a bit of coarse salt, and black pepper. Add 2 small ladlefuls of good chicken stock to deglaze the pan, stirring to meld flavors and make a nice glaze over the mushrooms.
While still in the pan, season the mushrooms with sea salt, and add the olives and Speck. Finally, add the oregano leaves, and toss together. The mushrooms are great served warm, but they can also be great at room temperature. If not serving right away, cover the pan with foil. Just before serving, remove the foil and arrange on a nice serving platter.
* Speck is a salt-and-cold-smoke cured ham that is a fusion of the much sharper smoked meats of middle Europe, and salt-cured, air-dried prosciutti of northern Italy.